Effects of bedroom environment on average heart rate during sleep in temperate regions: Winter conditions in healthy males in their twenties with average BMI

The nocturnal average heart rate correlates more strongly with mortality rates than the resting heart rate or 24-hour average heart rate. Additionally, it has been suggested that as the average heart rate decreases, SDNN increases, indicating favorable autonomic function. This study aimed to identify the environmental factors that most significantly affect the day-to-day variability of average heart rate during sleep (SHR) in winter. Measurements were conducted in the participants' usual bedrooms of nine healthy male participants in their twenties with an average BMI, living in a temperate region. The measurement periods were from December 1, 2022, to March 8, 2023, and from December 1, 2023, to February 6, 2024. In addition to the heart rate, body movement, room, radiant, and bed microclimate temperatures, carbon dioxide concentration, relative humidity, and illuminance were measured. The results demonstrated that, when comparing across participants, the average room and radiant temperatures during the measurement period had a significant negative correlation with the average SHR during the measurement period, with correlation coefficients of −0.83 and −0.91, respectively. Using a multilevel structural equation model, no valid model was obtained at the between level, which examined differences between participants, while a valid model was found at the within level, which examined differences within participants. At the within level, the most explanatory factors for SHR were body movement, operative temperature, bed microclimate temperature, carbon dioxide concentration, and the interaction term between the bed microclimate temperature and carbon dioxide concentration. Overall, a deeper understanding of the impact of environmental conditions on average sleep heart rate could facilitate the design of environments favorable for autonomic nervous system activity during sleep.